Re: [PATCH v6 5/5] riscv: Add and use alignment-ignorant memcpy

[Richard Henderson <richard.henderson@linaro.org>]

On 8/3/23 11:42, Evan Green wrote:
> On Thu, Aug 3, 2023 at 10:50 AM Richard Henderson
> <richard.henderson@linaro.org> wrote:
>> Outside libc something is required.
>>
>> An extra parameter to ifunc is surprising though, and clearly not ideal per the extra
>> hoops above.  I would hope for something with hidden visibility in libc_nonshared.a that
>> could always be called directly.
> 
> My previous spin took that approach, defining a
> __riscv_hwprobe_early() in libc_nonshared that could route to the real
> function if available, or make the syscall directly if not. But that
> approach had the drawback that ifunc users couldn't take advantage of
> the vDSO, and then all users had to comprehend the difference between
> __riscv_hwprobe() and __riscv_hwprobe_early().

I would define __riscv_hwprobe such that it could take advantage of the vDSO once 
initialization reaches a certain point, but cope with being run earlier than that point by 
falling back to the syscall.

That constrains the implementation, I guess, in that it can't set errno, but just 
returning the negative errno from the syscall seems fine.

It might be tricky to get a reference to GLRO(dl_vdso_riscv_hwprobe) very early, but I 
would hope that some application of __attribute__((weak)) might correctly get you a NULL 
prior to full relocations being complete.


> In contrast, IMO this approach is much nicer. Ifunc writers are
> already used to getting hwcap info via a parameter. Adding this second
> parameter, which also provides hwcap-like things, seems like a natural
> extension. I didn't quite follow what you meant by the "extra hoops
> above".

The check for null function pointer, for sure.  But also consider how __riscv_hwprobe is 
going to be used.

It might be worth defining some helper functions for probing a single key or a single 
field.  E.g.

uint64_t __riscv_hwprobe_one_key(int64_t key, unsigned int flags)
{
   struct riscv_hwprobe pair = { .key = key };
   int err = __riscv_hwprobe(&pair, 1, 0, NULL, flags);
   if (err)
     return err;
   if (pair.key == -1)
     return -ENOENT;
   return pair.value;
}

This implementation requires that no future hwprobe key define a value which as a valid 
value in the errno range (or better, bit 63 unused).  Alternately, or additionally:

bool __riscv_hwprobe_one_mask(int64_t key, uint64_t mask, uint64_t val, int flags)
{
   struct riscv_hwprobe pair = { .key = key };
   return (__riscv_hwprobe(&pair, 1, 0, NULL, flags) == 0
           && pair.key != -1
           && (pair.value & mask) == val);
}

These yield either

     int64_t v = __riscv_hwprobe_one_key(CPUPERF_0, 0);
     if (v >= 0 && (v & MISALIGNED_MASK) == MISALIGNED_FAST)
       return __memcpy_noalignment;
     return __memcpy_generic;

or

     if (__riscv_hwprobe_one_mask(CPUPERF_0, MISALIGNED_MASK, MISALIGNED_FAST, 0))
       return __memcpy_noalignment;
     return __memcpy_generic;

which to my mind looks much better for a pattern you'll be replicating so very many times 
across all of the ifunc implementations in the system.


r~